Non‐destructive evaluations of water uptake in epoxy coating

A commercial epoxy coating was studied on steel panels and as free films to investigate water uptake using non‐destructive techniques (gravimetric analysis, Electrochemical Impedance Spectroscopy [EIS], Attenuated Total Reflection‐Fourier Transform Infrared Spectroscopy [ATR‐FTIR], and Scanning Acoustic Microscopy [SAM]) after immersion in 3.5 wt% NaCl solution. In epoxy free films, water uptake showed rapid absorption initially, slowing down due to the counteracting process between water uptake and leachable content. Later, a mass decrease indicated leaching dominance. On coated steel, EIS revealed rapid water absorption within 72 h, followed by slower uptake up to 336 h. The continuous increase in water content resulted from observed coating layer swelling, confirmed by SAM. ATR‐FTIR analysis indicated hydrogen bonding of most absorbed water molecules to the epoxy matrix. By combining non‐destructive techniques, water uptake mechanisms into the epoxy coating film were proposed.